The present invention relates to methods and apparatus for selectively changing a proportion of power provided by a solid state rectification circuit as compared with power provided by a vacuum tube rectification circuit, and delivering such power to an amplifier.
Designers of guitar amplifiers, up until the late 1950's, focused on creating inexpensive amplifiers with very little distortion. This was not an easy task as solid state devices were not generally available and nearly all amplifiers were designed and built using vacuum tubes. Although tube amplifiers inherently have more distortion than solid state amplifiers, the early amplifier designers tried to produce amplifiers with little or no distortion. Most of their designs had a high order of distortion, especially second harmonic distortion, because most used Class A power amplifier topologies.
In the context of electric guitar amplification, the existence of harmonic distortion in the amplifier is not undesirable. Indeed, it has been generally thought that even order harmonics are pleasing to the ear, (especially the second harmonic), while odd order harmonics are thought to sound unpleasant to some people, especially the third and fifth harmonics. The desirability of even order harmonics appears to hold even in fairly high ratios to the normal (undistorted) signal.
As most rock guitar players use solid body guitars, which provide relatively little resonance, most of the harmonics associated with the characteristic sounds identifiable as an electric guitar actually come from the amplifier and not the guitar. The harmonics introduced by the guitar amplifier differentiate one guitar system (e.g., guitar and amplifier combination) from another, thereby giving each system some individuality. Thus, guitar amplification has become a niche technology within the broader universe of the audio amplification arts. What sets a guitar amplifier apart from an amplifier found in, for example, a sound system or a home stereo is that the latter tends to reproduce sounds as closely as possible to the input signal, while the former significantly alters the sound and is, in essence, an extension of the instrument and a part of an overall guitar system.
Guitar amplifiers are typically designed with excess gain, allowing the guitar, when played hard, to generate a signal that is sufficient to overdrive one or more tube stages and thus generate deliberate gross distortion. The characteristics of the tube and the circuit design directly influence the tone that results.
It is noted that even the power supply can influence the tone of the system. For example, in many amplifier circuits, a high voltage AC source (such as from a mains transformer) is rectified using one or more tube rectifiers and smoothed using one or more rectification capacitors. The high level DC voltage from the rectification capacitors is used (among other places) to supply the plate(s) and/or screen(s) of the power output tube(s). When the rectification capacitors are undersized, in terms of their ability to hold up the rectified voltage during surges of output power from through the power output tubes, the high level DC voltage exhibits a characteristic “sag” and subsequent recovery. The magnitude of the voltage sag and recovery time of the sag characteristic may be considered musically engaging to many people.
Unfortunately, conventional amplifier circuits do not provide the user with a means to control or manipulate the characteristics of the B+ voltage sag and/or recovery in any smooth or continuous fashion.